[Dovecot] NFS mail storage

The parameters listed for nfs installations (mmap_disable, doctlock_use_excl, mail_nfs_storage, mail_nfs_index) - are they necessary for data integrity, and/or do they compensate for NFS latency and improve performance?

My understanding is the indexes are a critical part of a dbox storage, but on the other hand having local indexes is a major performance consideration. Is maildir a "better" choice under NFS than dbox?

Daniel